The present invention generally relates to semiconductor devices and more particularly to a mask having a patterned layer and a transparent layer that acts as an etching stop layer.
The photolithographic patterning is a fundamental process for fabricating semiconductor devices. In the photolithographic patterning process, a mask that carries thereon transparent and opaque device patterns is used for exposing a device pattern on a substrate. Conventionally, the resolution that can be achieved by the photolithographic patterning process has been limited to about 0.7 .mu.m in correspondence to the wavelength of the optical beam that is used for the exposure.
In the recent very large scale integrated circuits (VLSIs) having a much higher integration density, on the other hand, the technique of submicron patterning is essential for achieving the desired submicron resolution. Such a high integration density contributes to the increased operational speed of the logic devices. In the memory devices, the increased integration density leads to the increased memory capacity. For example, the current 4M-bit DRAMs require the design rule of 0.8 .mu.m. On the other hand, the so-called 16M-bit DRAMs require the design rule of 0.5-0.6 .mu.m. Further, the future 64M-bit DRAMs require the design rule of 0.3 .mu.m.
Currently, intensive efforts have been made to achieve the foregoing submicron patterning while using the conventional photolithographic patterning process. For example, the use of optical systems that have a larger numerical aperture is studied. By using the optical system having such an increased numerical aperture, one can increase the degree of resolution while using the conventional visible or ultraviolet optical beam. The use of the optical beam is particularly advantageous in view point of increasing the throughput of the exposure process. On the other hand, the use of optical systems having the increased numerical aperture raises a problem of decreased focal depth and various studies are made to eliminate the problem.
The use of so-called phase shift mask provides a preferable solution to the foregoing problems. In the phase shift mask, a transparent, phase shift pattern having a thickness determined by the wavelength of the optical beam, is provided to cancel the diffraction caused by the edge of the opaque device pattern formed on the mask. Generally, such a phase shift pattern is provided along the edge of the opaque pattern. When fabricating such a phase shift mask, it is necessary to pattern the transparent layer that forms the phase shift pattern while leaving the device pattern intact.
Conventionally, a silicon nitride film has been used as the etching stop layer that prevents further progress of the etching for protecting the device pattern. More specifically, the silicon nitride film is provided to bury the opaque device pattern underneath with a substantially flat upper major surface, and the transparent layer, typically of silicon oxide is provided on the upper major surface of the silicon nitride layer as the layer that forms the phase shift pattern. The phase shift pattern is thereby formed by etching the transparent layer, wherein the etching stops when the upper major surface of the silicon nitride is exposed as a result of the etching.
In the conventional phase shift mask thus fabricated, there exists a problem in that the silicon nitride layer absorbs the optical beam that is used for the exposure.
FIG. 1 shows the transmittance of a silicon nitride layer deposited on a silicon oxide substrate. In the currently used optical exposure system, the radiation with the wavelength of 436 nm in correspondence to the g-line of mercury is generally employed. In this wavelength, the silicon nitride layer can provide the transmittance of more than 60%. On the other hand, in the future optical exposure systems, the radiation having shorter wavelengths is going to be used. For example, the use of the radiation having the wavelength of 365 nm corresponding to the radiation of the i-line of mercury is studied. Further, the use of the radiation having the wavelength of 248 nm from the KrF excimer laser is studied. As can be seen in FIG. 1, the transmittance of the silicon nitride layer for these shorter wavelengths is less than 30%. This clearly indicates that the phase shift mask that has the silicon nitride etching stop layer cannot be used in the future optical exposure systems.